The present invention relates to failure analysis of integrated circuits (IC""s) or other multi-layer solid state devices, and more particularly to the removal of protective (passivation) layers to expose and diagnose underlying circuitry.
During the development process of new multi-layer semiconductor devices, such as integrated circuit devices (IC""s), many times the completed IC fails to operate properly. In such a case, the protective (passivation) layer that covers the underlying circuit(s) must be removed. Once exposed, the circuits are analyzed to determine which portion of the circuit was not properly formed (e.g. unwanted electrical short or open circuit). It is important to carefully remove the passivation layer in such a manner that the underlying circuits are not damaged.
Argon Ion, Xenon, Yag and Excimer lasers have been used in IC failure analysis to remove passivation layers. These lasers produce infrared, visible and ultraviolet light that is absorbed by the material used to form the passivation layer. This material is etched (evaporated) away by the laser output with sufficient precision in both location and depth as to expose the underlying circuits without damaging them.
One problem with using a laser system to etch IC materials is that each laser wavelength is ideal for different materials used to form the IC. Therefore, multiple laser wavelengths and systems may be needed depending upon the particular types of materials that are to be etched away.
Recently, SiO2 has become a preferred material used to form the passivation layer. Unfortunately, SiO2 is transparent to the infrared, visible and ultraviolet light produced by Ion, Yag and Excimer layers. Instead of etching the SiO2 material, the laser energy from these systems is transmitted through the passivation layer and causes direct damage to the underlying circuitry. Therefore, those performing failure analysis on IC""s using SiO2 have used focused ion beams to etch away the SiO2, where a beam of ions is generated and focused onto the passivation layer.
There are several drawbacks in using focused ion beams in IC failure analysis. First, systems that produce focused ion beams are expensive and cumbersome to use. Second, it takes a relatively long time for the focused ion beam to etch the proper amount of material from the SiO2 layer. Third, and most importantly, the ion beam etch process cannot be optically monitored in the same manner as the laser etching process. With laser etching, the laser beam location and etch process can be optically monitored with an off or on axis camera to ensure the proper amount of material is removed. With ion beam etching, the circuits must be mapped in advanced, and the ion beam directed to the protective layer over the circuits based upon such mapping.
There is a need for a system that accurately and precisely etches SiO2 material from IC""s without damaging the underlying circuits, while providing optical monitoring of the etch process. There is also a need for such a system to etch practically all types of materials used in modern integrated circuits, without having to swap laser systems or laser wavelengths.
The present invention solves the above mentioned problems by providing a laser system and method that utilizes the higher energy 157 nm line from a molecular fluorine (F2) laser to etch and machine IC materials, namely SiO2. The higher energy 157 nm wavelength is strongly absorbed by SiO2 and therefore can selectively remove the SiO2 layer without detrimentally affecting the metal conductors below.
The invention also provides visible red 718 nm radiation from the F2 laser which travels through upper SiO2 protective layer(s) to machine materials found in lower layers of the IC without disturbing the protective layer(s).
The present invention is a method of performing failure analysis upon a multi-layer semiconductor device. The method comprises the steps of exciting a gain medium containing molecular fluorine and disposed in a resonant cavity to generate an output beam having a wavelength around 157 nm, and directing the output beam onto a multi-layer semiconductor device to selectively etch away material therefrom.
In another aspect of the present invention, a failure analysis system includes a resonant cavity, a gain medium containing molecular fluorine and disposed in the resonant cavity, a power supply for exciting the gain medium to generate an output beam having a wavelength around 157 nm, and an imaging system that directs the output beam onto a multi-layer semiconductor device to selectively etch away material therefrom.
In yet another aspect of the present invention, the method of performing failure analysis upon a multi-layer semiconductor device includes the steps of exciting a gain medium containing molecular fluorine and disposed in a resonant cavity to generate an output beam having a wavelength around 157 nm, and directing the output beam onto a multi-layer semiconductor device that includes an integrated circuit covered by a passivation layer, wherein a portion of the passivation layer is etched away by the output beam to expose the integrated circuit.
In still one further aspect of the present invention, a method of etching a passivation layer formed on a semiconductor substrate using a beam of radiation having a wavelength of 157 nm generated from a molecular fluorine laser comprises the steps of directing the beam of 157 nm radiation towards the passivation layer, and selectively removing a portion of the passivation layer using the directed beam.